Sahlan, R., and M. R. B. Abdullah. “Perdagangan Luar Malaysia Dalam Konteks AFTA, NAFTA Dan EU: Pendekatan Model Graviti”. International Journal of Management Studies, vol. 14, no. 1, June 2007, pp. 167-98, https://e-journal.uum.edu.my/index.php/ijms/article/view/9825.